Flight distance and blood parasites in birds

Flight distance reflects the risk that an individual animal is willing to take when approached by a potential predator. Because residual reproductive value is the average number of offspring that an individual of a given age class is expected to produce after the current reproductive event, individuals with low residual reproductive value should take greater risks than the average individual to make them more likely to gain at least some reproductive success. Therefore, I predicted that individuals belonging to bird species with intense infections with virulent parasites to take greater risks than individuals of species with few or no virulent parasites. In a comparative study of mean flight distance of 133 different bird species, as estimated from the distance at which individuals fled when approached by a human, relative flight distance decreased with the number of blood parasite species and the prevalence of blood parasites, as expected if parasitism reduces residual reproductive value. Birds that take great risks in terms of reduced flight distance run elevated risks of mortality by predators that are allowed to approach potential prey. However, relative flight distance decreased independently for species richness and prevalence of blood parasites and for risk of predation due to the European sparrow hawk Accipiter nisus. These findings suggest that standardized measures of flight distance provide reliable information about risk taking by individuals, with important consequences for life history, parasitism, and risk of predation.